Photolysis of benzil in 2-propanol and in cumene

Abstract

Benzil has been irradiated in 2-propanol and in cumene with a mercury lamp filtered through Pyrex. The principal products are benzoin, 1,2,3,4-tetraphenyl-2,3-dihydroxybutane-1,4-dione (3), benzoin benzoate, benzaldehyde, benzoic acid, 1,2,3,4-tetraphenyl-2,3-dihydroxybutane-1,4-dione dibenzoate (5), and some unidentified high molecular weight compounds. The yield of 3 is strongly dependent on initial benzil concentration and temperature and on the solvent used. The quantum yield of benzil disappearance depends on initial benzil concentration to a lower limit which varies with solvent. At an initial benzil concentration of 0.083 moles/l, the quantum yield of benzil disappearance is 0.11 in 2-propanol and 0.018 in cyclohexane. Compound 3 does not appear to be formed by a coupling of the appropriate ketyl radicals, as in the benzophenone case but instead, a complex is formed between a benzil molecule and the ketyl radical. Compound 3 decomposes at temperatures as low as 25°. The principal path for the formation of benzaldehyde and benzoyl radicals is the dissociation of the ketyl radical. Excited benzil abstracts a hydrogen from 2-propanol at least ten times faster than it dissociates into two benzoyl radicals.

Energy transfer photocatalysis: exciting modes of reactivity

“…(d) It should not participate in side reactions with any substrate in its excited or ground state, e.g. hydrogen atom transfer or competing biradical reactions is common with ketone-based sensitizers, 27 photodecomposition of benzil, 28 etc .…”

A new electrochemical technique for the study of photochemical reactions in liquid media

“…The photochemical reactions of benzil in liquid solutions have been studied by several workers (16,19,22,37, 106, 140 In the above reactions, I is benzll, II is the ketyl radical. III is the ketyl radical anion, IV is monoprotonated stilbendiolate, V is the benzoin anion, k2 is a second-order rate constant, and ki is a first-order rate constant.…”